It is known that polysiloxane-polycarbonate block cocondensates have good properties with regard to low-temperature impact strength or low-temperature notched impact strength, chemical resistance and outdoor weathering resistance, and to ageing properties and flame retardancy. In terms of these properties, they are in some cases superior to the conventional polycarbonates (homopolycarbonate based on bisphenol A).
The industrial preparation of these cocondensates proceeds from the monomers, usually via the interfacial process with phosgene. Also known is the preparation of these siloxane cocondensates via the melt transesterification process using diphenyl carbonate. However, these processes have the disadvantage that the industrial plants used therefor are used for preparation of standard polycarbonate and therefore have a high plant size. The preparation of specific block cocondensates in these plants is often economically unviable because of the smaller volume of these products. Moreover, the feedstocks required for preparation of the cocondensates, for example polydimethylsiloxanes, impair the plant, since they can lead to soiling of the plant or of the solvent circuits. In addition, toxic feedstocks such as phosgene are required for the preparation, or these processes entail a high energy demand.
The preparation of polysiloxane-polycarbonate block copolymers via the interfacial process is known from the literature and is described, for example, in U.S. Pat. Nos. 3,189,662, 3,419,634, DE-B 3 34 782 and EP 122 535.
The preparation of polysiloxane carbonate block copolymers by the melt transesterification process from bisphenol, diaryl carbonate and silanol end-terminated polysiloxanes in the presence of a catalyst is described in U.S. Pat. No. 5,227,449. The siloxane compounds used are polydiphenyl- or polydimethylsiloxane telomers with silanol end groups. It is known, however, that such dimethylsiloxanes having silanol end groups, in contrast to diphenylsiloxane with silanol end groups, have an increasing tendency to self-condensation with decreasing chain length in an acidic or basic medium, such that incorporation into the copolymer as it forms is made more difficult as a result. Cyclic siloxanes formed in this process remain in the polymer and have an exceptionally disruptive effect in applications in the electrical/electronics sector.
U.S. Pat. No. 5,504,177 describes the preparation of a block copolysiloxane carbonate via melt transesterification from a carbonate-terminated silicone with bisphenol and diaryl carbonate. Because of the great incompatibility of the siloxanes with bisphenol and diaryl carbonate, homogeneous incorporation of the siloxanes into the polycarbonate matrix can be achieved only with very great difficulty, if at all, via the melt transesterification process. Furthermore, the preparation of the block cocondensates proceeding from the monomers is very demanding.
EP 770636 describes a melt transesterification process for preparation of block copolysiloxane carbonates proceeding from bisphenol A and diaryl carbonate using specific catalysts. A drawback of this process is likewise the demanding synthesis of the copolymer proceeding from the monomers.
U.S. Pat. No. 5,344,908 describes the preparation of a silicone-polycarbonate block copolymer via a two-stage process in which an OH-terminated BPA oligocarbonate prepared via a melt transesterification process is reacted with a chlorine-terminated polyorganosiloxane in the presence of an organic solvent and of an acid scavenger. Such two-stage processes are likewise very demanding and can be performed only with difficulty in industrial scale plants.
Disadvantages of all these processes are the use of organic solvents in at least one step of the synthesis of the silicone-polycarbonate block copolymers, the use of phosgene as a feedstock and/or the inadequate quality of the cocondensate. More particularly, the synthesis of the cocondensates proceeding from the monomers is very demanding, both in the interfacial process and particularly in the melt transesterification process. For example, in the case of the melt process, a small relative underpressure and low temperatures have to be employed, in order to prevent vaporization and hence removal of the monomers. Only in later reaction stages in which oligomers with higher molar mass have formed can lower pressures and higher temperatures be employed. This means that the reaction has to be conducted over several stages and that the reaction times are accordingly long.
In order to avoid the above-described disadvantages, there are also known processes which proceed from commercial polycarbonates. This is described, for example, in U.S. Pat. Nos. 5,414,054 and 5,821,321. Here, a conventional polycarbonate is reacted with a specific polydimethylsiloxane in a reactive extrusion process. A disadvantage of these processes is the use of highly active transesterification catalysts which enable the preparation of the cocondensates within short residence times in an extruder. These transesterification catalysts remain in the product and can be inactivated only inadequately, if at all. Therefore, injection mouldings made from the cocondensates thus prepared have inadequate ageing characteristics, more particularly inadequate thermal ageing characteristics. Thus, the resulting block copolycarbonate is unsuitable for high-quality applications. Compared to a block copolycarbonate from the interfacial process, this product does not have the appropriate properties, such as ageing characteristics and mechanical properties.
DE 19710081 describes a process for preparing the cocondensates mentioned in a melt transesterification process proceeding from an oligocarbonate and a specific hydroxyarylsiloxane. However, the industrial scale preparation of oligocarbonates for preparation of relatively small-volume specific cocondensates is very costly and inconvenient. These oligocarbonates have relatively low molecular weights and relatively high OH end group concentrations. Frequently, these oligocarbonates, because of their short chain length, have phenolic OH concentrations above 1000 ppm. Such products are not normally commercially available and would therefore have to be produced specifically for the preparation of the cocondensates. However, it is uneconomic to operate industrial scale plants with the production of small-volume precursors. Moreover, such precursors, because of the impurities present in these products, for example residual solvents, residual catalysts, unreacted monomers etc., are much more reactive than high molecular weight commercial products based on polycarbonate. For these reasons, corresponding precursors or aromatic oligocarbonates suitable for the preparation of such block cocondensates are commercially unavailable. Moreover, the process described in DE 19710081 does not allow preparation of block cocondensate within short reaction times. Both the preparation of the oligocarbonate and the preparation of the block cocondensate are effected over several stages with residence times totalling well over one hour. Furthermore, the resulting material is unsuitable for the preparation of cocondensates, since the high concentration of OH end groups and other impurities, for example catalyst residue constituents, lead to a poor colour in the end product.
None of the abovementioned applications describes a process which proceeds from conventional polycarbonates commercially available in principle and affords polysiloxane-polycarbonate block cocondensates in high quality.
High quality in this context means that the cocondensates can be processed in injection moulding or by extrusion processes and have a relative solution viscosity of preferably at least 1.26, more preferably at least 1.27, especially preferably at least 1.28, determined in dichloromethane at a concentration of 5 g/l at 25° C. using a Ubbelohde viscosimeter. Furthermore, the corresponding products must have a high melt stability. In addition, the products should not have any discoloration such as browning or yellowing.
Commercially available polycarbonates have only low reactivity and, in contrast to the above-described oligocarbonates or polycarbonate precursors, are very melt-stable. In other words, they can be compounded under the customary processing conditions or can be processed in injection moulding or in extrusion without restriction and without any change in the properties. The person skilled in the art thus assumes that these polycarbonates, which may also contain stabilizers or quenchers, are unsuitable for preparation of copolymers because of their high stability.